Here's why:

* Igneous rocks are formed from the cooling and solidification of magma (molten rock) or lava. This process happens deep within the Earth or on the surface, leading to different types of igneous rocks.

* Sedimentary rocks are formed from the accumulation and cementation of sediments, like sand, clay, or the remains of organisms. While important for the Earth's surface, they don't make up the primary building blocks of the crust.

* Metamorphic rocks are formed when existing igneous or sedimentary rocks are transformed by heat and pressure. They are important parts of the crust but are derived from the original igneous or sedimentary rocks.

While all three rock types are important components of the Earth, igneous rocks represent the foundation from which the crust is built.